In compliance with a recent increasing request for a highly dense package and high integration of a large scale integrated circuit (hereinafter referred to simply as "LSI"), it has become essential for a wiring pattern to be finely formed. One of the method of finely forming the wiring pattern is to utilize an exposure light source with a short wavelength to be for lithography. That is, an excimer laser (far ultraviolet ray such as KrF laser: 248 nm, ArF laser: 193 nm or the like, F.sub.2 laser: 157 nm, Ar2 laser: 121 nm, soft X-ray: 13 nm or the like) which has a shorter wavelength than an ultraviolet ray such as g ray (436 nm), i ray (365 nm) or the like, an electron beam or an X ray has increasingly been utilized. In face, a mass production of the pattern utilizing an excimer laser source of 248 nm KrF excimer laser source has already started. Moreover, in a lithography technology of 0.15 .mu.m or less corresponding to the design rule of 4 Gbit DRAM, it is essential to mass produce a lithography (ArF exposure technology) using 193 nm ArF laser source which has a shorter wavelength than 248 nm KrF laser. The ArF exposure technology has at first started with a monolayer resist process employing an organic alicyclic-based resist. However, since a problem such as a fall down of a pattern caused by surface tension of water with increase of aspect ratio at the time of wet development, an influence of a step on a substrate, decrease of focal depth or the like is generated in 0.13 .mu.m or less, it has become difficult for the pattern to be formed with the conventional monolayer resist process.
In order to solve the above problems, a two-layer resist process for performing resolution of only the resist surface has been proposed and is anticipated to replace the conventional process rapidly. In the two-layer resist process, a material which is easily dry etched by oxygen plasma such as phenolic novolak resin, cresol novolak resin or the like is first applied by a spin coating method on the substrate followed by flattening a surface of the substrate; then the pattern is formed by means of a resist which is resistant to dry etching. Thereafter, the thus formed pattern is transfered to a lower layer by anisotropy etching by means of oxygen plasma. This process is performed under the dry development so that there exists no influence of surface tension of water whereupon the resist pattern having a high aspect ratio can be obtained without causing a fall down of a pattern. This two-layer resist process requires a resist material which is resistant to oxygen plasma etching. The resist material having a silicon atom is excellent in oxygen plasma resistance; for this reason, various resist materials having a silicon atom have heretofore been reported. Particularly, ladder-type polyorganosilsesquioxane has favorable characteristics such as high silicon content, excellent thermal stability, excellent oxygen plasma resistance and the like so that development of an Si-based resist material using this compound has been actively tried.
Moreover, high resolution corresponding to a processing size as well as sensitivity enhancement has increasingly been required for the resist material which undergoes LSI fine processing. This is because it is necessary for load put on a laser exposure device to be decreased in a manner that, since life of a source gas used for excimer laser oscillation is short and also light exposure resistance of a lens optical material is low, light energy to be incident on a projection optical system must be controlled and so forth.
As a method of sensitivity enhancement of the resist, a chemical amplified type resist which utilizes a light-causing acid generator that is one of sensitizers has been proposed. As an exemplary illustration, for example, a resist comprising a combination of triphenylsulfonium hexafluoro-arsenate and poly-tert-butoxycabonatephenol is described in Examined Published Japanese Patent Application (Kokoku) (hereinafter referred to simply as "JPB") No. 2-27660 (U.S. Pat. No. 4,491,628, EP No. 102450). The chemical amplified type resist has already been practically produced in quantity as a resist for KrF excimer laser. The chemical amplified type resist is characterized in that a proton acid generated from an acid generator by means of exposure is diffusingly spread by performing post exposure bake (hereinafter referred to simply as "PEB") after the exposure thereby catalytically amplifying chemical reaction of the resist resin by several hundred to several thousand times.
In such way, photoreaction efficiency (reaction for one photon) of the chemical amplified type resist has increased thereby attaining the tremendous sensitivity enhancement over the conventional resist so that it has become essential for resists with a short wavelength after KrF excimer laser to adopt a chemical amplification mechanism.
Also as an Si-based resist material, a resist material which utilizes this chemical amplification mechanism has been proposed. For example, it has been reported that polyhydroxybenzylsilsesquioxane described in Unexamined Published Japanese Patent Application (Kokai) (hereinafter referred to simply as "JPA") No. 8-160620 (U.S. Pat. No. 5,612,170) comes to be a chemical amplified positive-type resist of by first protecting one of its hydroxyl groups with a tert-butoxycarbonyl group (hereinafter referred to simply as "t-BOC") and secondly combining the resultant resist with an acid generator.
However, this polyhydroxybenzylsilsesquioxane contains an aromatic ring which absorbs 193 nm ArF laser wavelength (when its coating thickness is 1 .mu.m, transmission ratio being 60% or less) so that it is difficult for the compound to form a pattern with high sensitivity and high resolution as a resist for exposure with light having a wavelength of 193 nm or less.
On the other hand, as an example of an Si-based resist material which does not contain an aromatic ring, ethylcarboxylpolysilsesquioxane has been proposed (JPA-5-323611). This ethylcarboxylpolysilsesquioxane can be obtained by performing hydrolysis condensation of hydrosilylated tert-butyl methacrylate under an alkali catalyst. However, all carboxyl groups of side chains are protected (with t-BOC) so that it is difficult to enhance its sensitivity unless a large number of protected groups are decomposed in order to render its exposured portion soluble to an alkali developer. Even if a large number of the protected groups should be decomposed, it has a disadvantage that hardening compressive stress of a resist coating is increased whereupon a crack or separation may occur. Therefore, it can not be a resist material suitable for fine processing.
As another example, polyhydroxycarbonylethyl-silsesquioxane has also been proposed (JPA-8-160623). However, in this polyhydroxycarbonylethyl-silsesquioxane, though a portion of carboxylic groups of side chains is protected with a tert-butyl, t-BOC, tetrahydro-pyranyl group or the like, protection ratio at this time is as low as 10 to 25% so that quantity of carboxylic acids existing in its unexposed portion is large, which causes its solubility to the alkali developer is too high. Moreover, it is difficult to raise the protection ratio. For this reason, if a standard developer, namely, an agueout solution of 2.38% tetramethylammonium hydroxide (hereinafter referred to simply as "TMAH") is used, a developing characteristic is bad. There exists a disadvantage that the pattern resolution can not be performed unless it is used after diluted by about 20 times. Therefore, it, in fact, is difficult to practically adopt it to a mass production process.
Recently, an Si-containing acrylic copolymer has been proposed in JPA-9-110938 (EP No. 758102). This comprises a terpolymer (copolymer) containing an acrylic monomer having a group decomposable with an acid such as a tert-butyl, t-BOC group or the like and another acrylic monomer having a siloxane portion such as methacryloxypropyltris(trimethylsiloxane)silane or the like. However, these copolymers have introduced a siloxane structure as the Si portion so that it is difficult to raise Si content in the resist resin. Hence, an etching rate with a selection ratio of 10 or more which is an indicator of oxygen plasma resistance required for the Si resist can not be achieved.
Moreover, there exists a disadvantage such that, since the siloxane portion, is highly hydrophobic, it has a low wettability to a 2.38% TMAH developer whereupon a paddle-type development device which performs development by placing the developer on a wafer while keeping it still thereon can not be employed. The paddle-type development device consumes a relatively small quantity of developer as well as effectively reduces a resist residue and tailing so that it has primarily been employed in developing a positive-type photoresist in recent years; hence, it is difficult for this resist to be commercially mass produced.
Therefore, a novel material that is an Si-based resist material of chemical amplified type, does not absorb light with a wavelength of 193 nm or less, is adaptable to the 2.38% TMAH developer, has a good oxygen plasma resistance and does not produce a problem such as pattern insolubilization, swelling or the like has seriously been desired.